A digital watermark is regarded as a technology for embedding additional information which a person is not aware of, with respect to image data and sound data. By embedding copyright information contents and purchase information of a user, it is possible to specify the presence or absence of the copyright information and the originator of the leak when the contents are illegally leaked.
Also, in a case in which an image and movie contents are the targets to embed the digital watermark, the digital watermark may be applied to a digital signage. By detecting the digital watermark by a mobile phone or the like with a camera from a movie advertisement displayed at a street screen or a television set, it is possible to acquire additional detailed information and the like of a product.
The contents, in which the digital watermark is embedded, may be degraded after compression and cutting of data, a filtering process, and the like. Thus, it is preferable to have prevention with respect to conceivable degradation factors. Exceptionally, in a case of using the digital watermark in the purpose of copyright protection, the contents, in which the digital watermark is embedded, are preferred to be capable of preventing attacks by a malicious user, the attacks which analyze and invalidate the digital watermark.
These attacks are called “collusion attacks”. The “collusion attacks” are regarded as attacks in which multiple users bring their contents in which the digital watermark is embedded and compare portions where data are different, so that a targeted digital watermark is analyzed and invalidated.
As a typical collusion attack, an averaging attack is known. In the averaging attack, the target contents are attacked by averaging multiple contents including the digital watermark and generating new contents in which the digital watermark is degraded. It is assumed that there is a digital watermark indicating a bit “0” at a location in the contents of a user A and there is a digital watermark indicating a bit “1” at the same location in the contents of a user B. In a case in which two users collude with each other to perform the averaging attack, the bit “0” of the user A and the bit “1” of the user B at the location are averaged, and the digital watermark at the location is erased.
As described above, users colluding with each other target the location in the contents in which watermark information is different. In general, the more users who collude with each other, the wider success of the attack. The following two technologies are known as related arts to resist the collusion attack described above.
A first technology is related to embedding the digital watermark using collusion-secure codes. In this technology, codes (c-secure with ε error) are presented, so-called “collusion-secure codes”, in which probability for erroneously detecting a user who does not collude becomes ε in a case of c users colluding with each other.
A code length to generate the collusion secure code is conceptually indicated to be an order of c (counts of users) raised to the fourth power (c4). Also, there is a technology to realize the collusion-secure codes by the code length which is an order of c raised to the second power (c2). There are approaches to reduce code length of the collusion-secure codes.
A second technology is related to embedding the digital watermark in which watermark data are diffused by a key in a different method for each user. In this technology, it is realized to diffuse the watermark data by scrambling based on the key. The digital watermark being embedded is detected by restoring data which are diffused by the key used to embed the digital watermark. In a case of the collusion attack, it is possible to conduct a random noise for the watermark information of the user which does not correspond to the key when the digital watermark is detected. Thus, it is possible to realize watermark embedding having the collusion secure.